1. Field of the Invention
The present invention relates to a method and device for attaching a strip tape to a packaging web so as to close a liquid pouring hole formed in the web.
2. Prior Art
Currently, a paper container called a pack container for a liquid product such as milk, juice etc. as shown in FIG. 10 is used. In order to easily pour juice or milk from this kind of pack container P, an opening device is provided such that a liquid-pouring punch hole 5 is previously formed on the container top plane and its inner plane is sealed with a strip tape 2 to make the container liquid-tight, and a pull-tab 7a is heat-sealed from outside so that the liquid pouring hole 5 is easily opened by peeling off the pull-tab 7a together with the strip tape 2.
Conventionally, in mounting this kind of opening device onto a container, the opening device is formed at a prescribed position on the packaging web, and the packaging web is folded to make the container into which liquid food is filled. As shown in FIG. 1, the packaging web 1 usually consists of a paper material 1.sub.1, which is a supporting material layer, on one surface of which is fixed a metallic foil layer 1.sub.2 such as aluminum foil, and the surface of the metallic foil layer 1.sub.2 and another surface of the paper 1.sub.1 are laminated with a polyethylene layer 1.sub.3 (Note, in the figure, the web is drawn to appear thicker to make it easier to see.)
In forming an opening device for such a packaging web 1, when sealing the liquid-pouring punch hole 5 with a resin strip tape 2, the strip tape 2 is placed over a prescribed position on the packaging web, heated and deposited by a heat coil for high-frequency induction heating onto the surface of the packaging web 1 (refer to FIG. 7) in which punch holes 5 are intermittently and successively formed after each prescribed interval 1. This high-frequency induction heating coil is a quasi-rectangular print coil 10.sub.4 of a prescribed length (corresponding to the intermittent conveyance distance for the packaging web) on a flat plate having an open coil part 10.sub.1 that surrounds a punch hole 5 in the center, as shown in the example in FIG. 4 (except the central hole 10.sub.3). A rectangularly formed coil part 10.sub.2 is formed on both sides of the open coil part 10.sub.1. Between the high-frequency induction heating coil 10 and the pressure rail 11, the strip tape 2 is pressed onto the packaging web 1 and the metallic foil layer 1.sub.2 adjacent to the tape is heated by a high-frequency wave to deposit the tape onto the web. When heating and depositing the tape onto the web plane, the punch hole 5 of the packaging web 1 is positioned inside the open coil part 10.sub.1 in the center of the quasi-rectangular print coil 10.sub.4 on the high-frequency induction heating coil 10. After the strip tape 2 closes the punch hole 5 on the polyethylene-laminated layer 1.sub.3 surface on the metallic foil layer 1.sub.2 side of the packaging web 1 and is deposited with a prescribed margin for longitudinally sealing the web (see FIG. 8), a pull-tab 7a of a prescribed length closes the punch hole 5 on the polyethylene-laminated layer 1.sub.3 surface of the paper material 1.sub.1 on the back of the deposited part, and are heat sealed together with the strip tape 2 onto the packaging web 1 to form the opening device (see FIG. 9).
When the strip tape 2 is pressed onto a packaging web 1 to close a punch hole 5 and deposited by high-frequency induction heating between the high-frequency induction heating coil 10 and the pressure rail 11, there is no metallic foil layer 1.sub.2 or polyethylene-laminated layer 1.sub.3 inside the edge of a punch hole 5. When the coil part 10.sub.1 which surrounds the central punch hole 5 of the spring coil part 10.sub.3 heats the edge of the punch hole 5 of the metallic foil layer 1.sub.2, the edge of the laminated layer 1.sub.3 and the strip tape 2 contacts the part that is heated to a half-melted condition. A part of the strip tape 2, that is positioned between the unheated portion, which is inside the unheated punch hole 5, and the heated portion, becomes thin, resulting in pinholes easily forming inside the strip tape, thus causing leakage and spoilage of the contents. Furthermore, the edge of the punch hole is subject to an intense load, and this, together with heating of the edge of the punch hole of the strip tape, tends to result in poor strength at the joint at the edge of the punch hole.